Apparatus for obtaining uniform droplets

ABSTRACT

THE DISCLOSURE RELATES TO A METHOD AND APPARATUS FOR FORMING DROPLETS OF SUBSTANTIALLY CONSTANT SIZE FROM A LIQUID, WHEREIN THE SAID LIQUID IS PASSED THROUGH A PERFORATED WALL, AND USING VIBRATIONS TO DIVIDE THE LIQUID INTO DROPLETS, THE SAID METHOD BEING CHARACTERIZED IN THAT THE LIQUID ISSUES FROM THE PERFORATIONS IN THE FORM OF LAMINARY JETS, THE PLATE IS VIBRATED AT ONE OF THE RESONANCE FREQUENCIES THEREOF, THE STATIONARY WALL IS MAINTAINED IN ONE OF ITS NODAL ZONES SO THAT IT IS CONTINUOUSLY SUPERPOSED UPON THE LAMINAR FLOW OF THE PERIODIC PERTURBATIONS WHICH DIVIDE THE JETS INTO UNIFORM DROPLETS. AMONG THE APPLICATIONS OF SAID METHOD, MAY BE MENTIONED THE MANUFACTURE OF GRANULATED CHEMICAL FERTILIZERS (PURE OR MIXED UREA-AMMONIUM NITRATE, CALCIUM NITRATE, COMPLEX FERTILIZERS, ETC.), BUT THE METHOD CAN BE USED JUST AS ADVANTAGEOUSLY IN ALL CASES WHERE IT IS DESIRED THAT SOLID GRANULATED PRODUCTS SHOULD BE OBTAINED BY THE DISPERSAL OF A LIQUID, OR A PRODUCT WHICH CAN BE SOLIDIFIED BY COOLING OR DRYING LIQUID DROPLETS (AS IN THE MANUFACTURE OF SHOT FOR INSTANCE).

y 25, 19.71 A. M. KALTENBACH 3,579,721

APPARATUS FOR OBTAINING UNIFORM DROPLETS Filed 001;. 18, 1968 & FIG! 1-75,;

7 2 U v m A A [5 o A -o Willi/[film O ELECTRODE 0 M x kalfenfiazh United States Patent 3,579,721 APPARATUS FOR OBTAINING UNIFORM DROPLETS Roger Max Kaltenbach, 3 Avenue Erlanger, Paris, France Filed Oct. 18, 1963, Ser. No. 768,679 Claims priority, application Switzerland, Oct. 19, 1967, 14,648/67 Int. Cl. B2911 23/08 US. Cl. 182.7

ABSTRACT OF THE DISCLOSURE The disclosure relates to a method and apparatus for forming droplets of substantially constant size from a liquid, wherein the said liquid is passed through a perforated wall, and using vibrations to divide the liquid into droplets, the said method being characterized in that the liquid issues from the perforations in the form of laminary jets; the plate is vibrated at one of the resonance frequencies thereof; the stationary wall is maintained in one of its nodal zones so that it is continuously superposed upon the laminar flow of the periodic perturbations which divide the jets into uniform droplets. Among the applications of said method, may be mentioned the manufacture of granulated chemical fertilizers (pure or mixed urea-ammonium nitrate, calcium nitrate, complex fertilizers, etc.), but the method can be used just as advantageously in all cases where it is desired that solid granulated products should be obtained by the dispersal of a liquid, or a product which can be solidified by cooling or drying liquid droplets (as in the manufacture of shot for instance).

The invention relates to improvements in the method of obtaining uniform droplets and the manufacture of granulated products.

The method consisting in vibrating a filiform liquid flow to decompose it into uniform droplets has been used for many years. Mechanical vibrations of relatively low frequencies operating with banks or plates provided with spraying nozzles have been used for this purpose. The frequencies used were generally about 50 cycles/ second, but necessitated a very large expenditure of power as it meant vibrating a unit having considerable inertia. The spraying systems have been vibrated by means of high power, low frequency, electromagnetic or electromechanical vibrators. The method of applying sound vibrations to fluid streams after they emerge from jets formed in a perforated plate is also known but, under these conditions, the sound vibration moves heterogeneously along a substantial distance within the sound-pipe surrounding the jets, and so providing an obstacle to homogeneous action. Furthermore, to moderate the effect of sound waves on the plate itself, a support plate has been disposed beneath the perforated plate to abolish, or at least lessen, the effect on the said perforated plate, of the sound chambers wherein stationary vibrations with knots or bulges are established, so that any jet coinciding with a knot will not be vibrated and that the division of said jet will be aberrant.

Teachings of the known art further indicate, firstly, that the size of the droplets and the range of sizes they cover are a function of the diameter of the nozzle, fluid pressure, amplitude of the vibration, concentration and temperature of the liquid (when these factors are constant droplet size is very uniform), and secondly, that the range of droplet sizes is limited by high values of acceleration.

Generally speaking, a number of vibrations in the range of between about 50 and 1200 per second has been suggested up to the present time.

During his research the applicant has established that 14 Claims 1 3,579,72i Patented May 25, 1971 the formation of homogeneous droplets from thin jets of liquid was very substantially improved, and made more reliable and less costly, by applying these vibrations, frequencies in all cases higher and preferably very much higher to those now used, directly to the perforated plate so as to cause this plate to vibrate at one of its resonance frequencies, and maintaining the said plate, or wall stationary in one of the nodal zones thereof. In other words, the invention provides that only the perforated Wall 0 through which the liquid for forming the droplets passes is vibrated, to the exclusion of the support and liquid feed means.

A first object of the invention is, therefore, a method for forming droplets of substantially constant size from a liquid, wherein the said liquid is passed through a perforated wall, and using vibrations to divide the liquid into droplets, the said method being characterized in that the liquid issues from the perforations in the form of laminary jets; the plate is vibrated at one of the resonance frequencies thereof; the stationary wall is maintained in one of its nodal zones so that it is continuously superposed upon the laminar flow of the periodic perturbations which divide the jets into uniform droplets.

In other words, according to the invention, one side of a perforated vibratory resonance wall, secured in a nodal zone, is supplied with liquid so as to form a laminar jet of liquid issuing from each orifice so that this jet is modulated at a frequency equal to the vibrational frequency of this wall, which thereby separates the jet into droplets. The frequency of the vibrations applied to the plate is greater than 1000 cycles per second, and preferably between about 1250 and 12,000 c./sec., so as to abolish the influences of vibrations exterior to the system which might perturb the formation of uniform droplets. The size of the droplets depends upon a certain number of factors, such as frequency of the vibrations of the plate, size and form of the plate openings, fluid flowing speed, viscosity, surface tension which are taken into consideration to determine the vibration to be used.

According to another characteristic of the invention dispersion of the uniform droplets formed at the plate outlet is carried out in any suitable known way. For instance, an electrostatic field can be applied by means of an electrode charged with respect to the plate and disposed adjacent to the locus where the droplets are formed, this having the effect of dispersing these droplets as they all have an electric charge of the same sign, and, consequently, repel one another. Furthermore, the fact that each droplet is so charged makes it possible toexercise an influence upon its trajectory in any desired manner and, amongst others, by means of a suitable magnetic or electric field. It is thus possible, for instance, either to disperse the droplets to a greater extent so that the enclosure into which they are projected is better filled, or to prevent them from knocking against this enclosure by applying a suitable electric charge to these walls.

According to another characteristic it is furthermore possible to apply additional mechanical vibrations to the perforated plate or to the support or feed devices of this plate instead of the dispersion means such as the aforesaid electric or magnetic fields. The frequency of these additional vibrations is preferably transversal with respect to the direction of the jets, and varies from 25 to 1000 cycles/ second.

The invention also relates to an apparatus for putting the method according to the invention into effect. This apparatus comprising a circular plate with at least one orifice, liquid feed means above the said plate, and vibration means, is characterized in that the vibration means are connected to this plate and arranged so as to cause it to vibrate continuously at one of the resonance frequencies thereof, fixing means being provided to sealingly connect the feed means to a nodal zone of the said plate, thus preventing the transmission of vibrations.

The plate vibrating means can consist either of a pneumatic vibrator, or preferably, of an electrodynamic vibrator. The advantage of the latter solution is firstly, that it is easy to adjust the vibration frequency of the electromagnetic vibrator so that it is tuned to the natural frequency of the plate, this entailing a minimal consumption of power, and secondly, to cause large plates to vibrate and consequently to obtain high output spraying jets.

The frequency of the vibrator is selected so that it is essentially equal to the desired resonance frequency of the plate. In the case of a circular plate it is preferable to select a mode of vibration having a singly nodal circle of a diameter of about 0.675 times the diameter of the plate. The circular plate is then secured to its support, which is integral with the liquid feed device, by means of clamping and sealing joints disposed on the said nodal circle. The energization, which is tuned to the desired resonance frequency of the plate, can be applied to a peripheral zone of the plate, that is to the exterior of the nodal circle, or better, to the center thereof.

The perforated plate can be planar or convex with apertures parallel to the axis of the plate, or inclined with respect to this axis so as to provide parallel or diverging jets of droplets. Means can be provided to vary the orientation of the perforated, vibratory plate to direct the jets in any desired direction.

The speed of liquid flow to the orifices or holes of the form of said orifices, and the nature and size of the liquid feed means, are selected to produce a laminar jet along a distance sufiicient so that the perturbations produced by the vibrations may spread, in crossing relationship, until the droplets separate. The diameter of the orifices is selected as a function of the desired size of the droplets.

According to other characteristics of the invention the sized droplets are cooled to solidfy them, to improve the granulation methods now in use in which a granulation tower with a vertical axis (or prilling tower) is used. The present method, which provides perfectly sized and uniform granules from uniform droplets, substantially abolishes any dust formation and the necessity of recycling fines and granules that are too big. Furthermore, the present method enables a better transversal distribution of droplets in the tower to be obtained, particularly when the aforementioned dispersal means are used. This uniformity and satisfactory distribution of droplets is a substantial help in cooling, thus enabling the height of conventional towers to be reduced, which is obviously a considerable advantage.

When, according to the above characteristic, the method according to the invention is used to manufacture granulated products, a fairly high pressure, in the range of 1 to 5 kg./cm. and therefore much higher than the pressures used in conventional granulation towers, can be applied to the liquid feed; it is therefore possible to communicate a high initial speed to the droplets and a flat trajectory, and even a horizontal flat trajectory, to the jets as a whole.

It is particularly advantageous to dispose the dropletforming system according to the invention at the bottom of the tower, or the structures such as a granulation passage replacing same. The angle of inclination of the jets to the horizontal is then preferably selected to provide the droplets with a maximum period of flight. Under these conditions, according to another characteristic, the liquid is not sprayed in the upper portion of a granulation tower as at present, but in the lower portion of the structure in which the jet of droplets is projected. The high initial speed of the droplets, associated with their dispersal by a suitable electrostatic field for instance, enables a very satisfactory heat exchange coefficient with the surrounding air to be obtained. This form of embodiment of the method of the invention is therefore more efficient and much less costly than the known arrangement of a conventional granulation tower, and has none of the drawbacks of the latter, in which the hot liquid to be granulated is distributed by means of non-vibrating perforated plates located in the upper portion of the tower. In this known arrangement the liquid pressure at the nozzle inlet should remain low so that the horizontal range of the jet of droplets is no greater than the diameter or length of the tower. Such a range is, therefore, necessarily limited, and the droplets are distributed very irregularly across a horizontal section of the tower.

The industrial applications of the method according to the invention are, of course, extremely numerous; they apply to practically all cases where it is desired to reduce a liquid into identical droplets, and cool the said droplets so as finally to obtain a uniform product consisting of perfectly sized spherical granules.

Among these applications may be mentioned the manufacture of granulated chemical fertilizers (pure or mixed urea-ammonium nitrate, calcium nitrate, complex fertilizers, etc.), but the method can be used just as advantageously in all cases where it is desired that solid granulated products should be obtained by the dispersal of a liquid, or a product which can be solidified by cooling or drying liquid droplets (as in the manufacture of shot for instance). The method is also used to advantage when a liquid such as water is to be contacted with a gas (refrigerants, condensers, etc.).

In the case of the production of pure or mixed ammonium nitrate or urea prills, spraying of the molten product at a very high concentration is now carried out in the top of a tower known as a prilling tower at a slightly higher temperature than the crystallization temperature of the product; this reduction of droplets of a hot liquid is carried out either by means of dynamic devices (rotating turbines or baskets), or by spraying nozzles using centrifugal force to disperse the liquid into droplets. All these devices have the major disadvantage of producing very irregular size droplets. The use of static spraying nozzles, with planar or convex perforated plates has become more and more frequent and permits of a substantial improvement in the granulation of the droplets produced, and consequently in the solid products obtained after cooling and solidifying, but this granulation is still far from being uniform. The method according to the invention, which provides a practically completely homogeneous granulation and abolishes dust is, therefore, of primary importance for such operations.

Various forms of embodiment of the invention are described hereinafter with reference to the appended drawing. In the drawing:

FIG. 1 is a diagrammatic view of a vibratory device according to the invention;

FIG. 2 is a form of pneumatic vibrator for use in a device according to the invention;

FIG. 3 shows, diagrammatically, the method of droplet formation by the method of the invention;

FIG. 4 is a diagrammatic view of the known spraying nozzle device on the top of a prilling tower;

FIG. 5 is a diagrammatic view of a granulation device according to the invention.

FIG. 6 is a detail of a convex plate having openings parallel to a plate axis.

FIG. 6A is a detail of a convex plate having openings inclined to a plate axis.

The device of FIG. 1 includes a perforated circular plate 1 comprising several orifices 2, a cylindrical liquid feed chamber 3 connected to an input conduit 4 provided with a valve 5 and a vibrator 6 integral with plate 1 by means of an arm 7.

Vibrator 6 is arranged so that the frequenty thereof is tuned to the desired resonance frequency of plate 1. The method of vibrating this last is selected so that it comprises a single circular nodal line along which plate 1 is secured to chamber 3 by means of clamping and sealing joints 8, 9 of a cheek and a ring 11, members :1 and 8 to '11 being maintained in contact by clamping means (not shown). Joints 8 and 9 are arranged so as to leave a sufificient space between plate 1 and cheek 10 on one side and ring 11 on the other in order to permit plate 1 to vibrate alone without vibrations being transmitted to feed chamber 3.

In the form of embodiment shown in cross-section on FIG. 2, vibrator 6 is of the pneumatic type and includes a vibrant member in the shape of a blade 12 forming a reed, secured at one end thereof to a plate 13- so as to cover an aperture 14 formed in the last. Plate 13 is secured on both sides to two air chambers 15, 16 by means of resilient sealing joints 17 and 18 respectively. A stream of air passing through aperture 14 from chamber 15 to chamber 16 to cause blade 12 to vibrate at its natural frequency. Vibration of blade 12 is transmitted by reaction to plate 13, the arm-forming portion 7 of which is integral with the perforated plate 1 (see FIG. 1). To ensure satisfactory operation of vibrator 6, chambers 15 and 16, which each form a column of air, are tuned to the natural frequency of the blade so that the assembly forms a pneumatic oscillating circuit.

The resiliency of joints 17 and 18 should be sufiicient to prevent the vibration of plate 13 diminishing to any considerable extent.

There is shown on BIG. 3 a perforated, vibratory plate 1 comprising a single axial orifice 2. Jet 19 emerging from orifice 2 has narrow regions that increase progressively until the droplets separate. An electrode 20 has the eifect of charging the jet negatively so that the droplets detached from it each carry a negative charge and repel one another so as to disperse. Magnet 3-1, further repels and directs the charged droplets.

FIG. 4 shows a conventional prilling tower 2 1 with normal spraying nozzles 22 firstly ejecting their jet 23 upwardly, and secondly 24 ejecting their jet 25 downwardly, a stream of air being forced into the bottom of tower 21 in the opposite direction to the final fall of the particles constituting the jets.

In the device shown diagrammatically in FIG. 5, a horizontal granulation chamber 26 is seen; at one end of the chamber is mounted a spraying bank 27 vibrated according to the invention (FIG. 1) and projecting granules 27' is a flat stream against the flow of a cooling air stream entering through apertures 28 formed in the other extremity of chamber 26, a vent hole 29 for this cooling air being provided in the end zone where bank 27 is positioned. The lower portion of chamber 26 being provided with a device 30, a conveyor belt in the example, for removing the cooled granules.

FIG. 6 illustrates a convex plate 32 with openings 33 oriented parallel to the plate axis. FIG. 6A shows a convex plate 34 with openings 35 inclined to the plate axis.

What I claim is:

1. Uniform droplet forming apparatus comprising a perforated plate with at least one opening extending therethrough, liquid feed means in communication with the plate for feeding liquid to the plate and through the at least one opening, and resonant frequency vibrating means connected to the perforated plate for vibrating the plate continuously at a resonant frequency thereof, thereby creating vibrating and nodal zones of the plate and causing liquid feeding through the at least one opening to form droplets, and sealing means connected to the feed means and to the plate at a nodal zone, for minimizing vibration transmission means between the plate and the feed means.

2. Apparatus of claim =1 further comprising electrode means positioned near the plate at a side thereof remote from the feed means for imparting electrostatic charges to the droplets.

3. Apparatus of claim 2 further comprising a magnet positioned adjacent the electrode remote from the plate for applying a magnetic field to the charged droplets for directing the course of the droplets.

4. Apparatus according to claim 1 wherein the vibrating means comprises a pneumatic vibrator.

5. Apparatus according to claim 1 wherein the vibrating means comprises an electrodynamic vibrator.

6'. Apparatus according to claim 1 wherein the plate is circular and has a single circular nodal zone with a diameter of about 0.675 times a diameter of the plate, and wherein the feed means includes a circular wall which is connected to the circular nodal zone.

7. Apparatus according to claim 1 further comprising means to vary orientation of the perforated vibratory plate.

8. Apparatus according to claim 1 in which the perforated plate is circular.

9. Apparatus according to claim 8 in which the perforated plate is convex with openings parallel to an axis of the plate.

10. Apparatus according to claim 8 in which the plate has openings inclined to an axis of the plate.

11. Apparatus according to claim "1 further comprising pressure means connected to the feed means for providing fluid to the plate at a superatmospheric pressure of about one to about five kilograms per square centimeter.

12. The apparatus according to claimv 1 further comprising a prilling enclosure surrounding the plate and feed means whereby liquid droplets are projected in the enclosure, and means connected to the enclosure for flowing cooling fluid in counter current relation to the fluid droplets, and means for transporting a product thereby formed.

13. The apparatus according to claim '12 wherein the prilling enclosure is horizontally elongated, wherein the plate and feed means are positioned near a first longitudinal end for projecting the droplets toward a second longitudinal end of the enclosure, and wherein the transporting means are positioned near the second longitudinal end. i

14. The apparatus according to claim v1 further comprising second vibrating means connected to the plate for applying additional mechanical vibrations to the perforated plate transversely to a direction of the feeding of liquid through the plate.

References Cited UNITED STATES PATENTS 2,251,727 8/1941 Wellech et al. l82.4X 2,392,072 1/ 1946 Vang.

2,859,560 11/1958 Wald et al. 1 82.4X 2,968,833 l/1961 Haven et al. 18-2.4X 3,274,642 9/ 1966 Cramer 18--2.7

I. SPENCER OVERHOLSER, Primary Examiner M. O. SUTTON, Assistant Examiner US. Cl. X.R. 264-9, l3 

